Rename cross-correlation analysis to cross-association analysis

[artur-sannikov]

When one searches for cross-correlation, they'll see that it's mostly related to time series correlations. That can become a font of confusion for beginners.

Might "cross-association" reflect better the book's idea? Or just simply call it "correlation analysis" because cross-association is also barely mentioned in multi-omics. I found one paper.

This term is used in chapters:

1. Community Typing (Clustering)
2. Multi-Assay Analyses
3. Exercises

[TuomasBorman]

I would go with "correlation analysis". That might be something that one might try to find from google

[artur-sannikov]

Agreed. I can open a pull request if approved.

[antagomir]

@artur-sannikov what's the status of this one?

[TuomasBorman]

Note that the structure of book has changed. Now we have chapters https://microbiome.github.io/OMA/docs/devel/pages/correlation.html and https://microbiome.github.io/OMA/docs/devel/pages/cross_correlation.html